1. Field of the Invention
The present invention relates to an electrostatic reproducing apparatus in which the ion transmission factor of an ion flow modulating electrode is enhanced to improve the reproducing speed.
2. Description of the Prior Art
An apparatus, in which the picture image signal of either the information generated by a computer, the information sent through communication lines or the information generated by converting the document image into electric signals by means of a solid state image sensor such as a CCD is reproduced on a sheet of reproducing paper, is known as an electrostatic reproducing apparatus, and its various types are proposed at present. In the electrostatic reproducing apparatus, an electrostatic latent image corresponding to the picture image signal is formed on a charge receptor such as a dielectric member or the reproducing paper. As one of the electrostatic latent image forming means, there is an electrostatic reproducing apparatus of the type, in which an ion flow modulating electrode for modulating the corona ions generated by a corona wire or the like in accordance with the picture image signal is used to form the electrostatic latent image on the dielectric member. FIG. 1 shows the schematic construction of one example of the electrostatic reproducing apparatus of the aforementioned type. As shown, either the picture image signal generated by photoelectrically converting the document image by means of a solid state image sensor such as the CCD or the electric information signal coming from a computer or communication lines is fed to a signal processing circuit 1, in which it is processed and is then fed to an ion flow modulating electrode drive circuit 2. Reference numeral 3 indicates a control circuit including a clock generator for imparting the timing at which the signal is derived from the signal processing circuit 1 and the drive circuit 2. An ion flow modulating electrodes 4 is constructed by sandwiching an insulating layer between a common electrode 4a and a control electrode 4b and is formed with a plurality of ion transmissive apertures 4c which are arranged in one or plural rows, and the picture image signal is fed from the drive circuit 2 to the control electrode 4b. Above the ion flow modulating electrode 4, there is arranged a corona ion generator 5 which has a built-in corona wire 5a. Below the ion flow modulating electrode 4, on the other hand, there is arranged a rotary drum 6 which is made of a dielectric material. Around the periphery of the dielectric rotary drum 6, there are arranged a developing device 7, a transfer electrode 8, a separation pawl 9, a discharge electrode 10, a developing agent cleaning blade 11 etc., all of which are necessary for the usual electrophotographic processing. A fixing device 12 is also provided. The ions generated by the corona ion generator 5 are controlled in their flow rate by the picture image signal to transmit through the ion transmissive apertures 4c, which is applied to the control electrode 4b of the ion flow modulating electrode 4, so that an electrostatic latent image corresponding to the picture image information to be reproduced is formed on the dielectric member on the rotary drum 6. The electrostatic latent image thus formed is developed at the developing device 7 by means of a developing agent into a toner image, which is transferred to a sheet of reproducing paper P by the action of the transfer electrode 8. After that, the reproducing paper P is separated from the dielectric rotary drum 6 by the action of the separation pawl 9 and is fixed by the action of the fixing device 12 until it is discharged to the outside of the apparatus.
Here, by the electrostatic reproducing method using the ion flow modulating electrode thus far described, a high quality picture image having a high resolution and tonal response characteristics can be reproduced while maintaining the electrode and the electrostatic charge receptor such as the dielectric member non-contacting (i.e., spaced at 0.1 to 2.00 mm). However, there arises a problem that the reproducing speed is low partly (1) because there is a limit to the current density of the ions generated by the ion generator and partly (2) because there is a lower limit to the ion control time due to the ion mobility. Therefore, the electrostatic reproducing method thus far described is applied at most to a printer using an ink mist. With this in mind, the main current of the electrostatic reproducing system at present uses a multi-stylus electrode. In order to augment the ion flow reaching the reproducing unit, therefore, it is conceivable to provide a method in which the voltage to be applied to the corona wire is raised. In the case of the electrostatic reproducing apparatus using a screen photosensitive member when the ion flow to be generated by the corona wire is increased however, the ion flow reaching the reproducing unit is reduced, or the picture quality deteriorates because an electric field into which the ions are to flow is formed at the side of the screen photosensitive member. For this reason, an upper limit is set to the voltage to be applied to the corona wire. In the case of the electrostatic reproducing apparatus using the ion flow modulating electrode, on the other hand, the ion flow generated by the corona wire 5a is directed to the ion flow modulating electrode 4 along the electric field, which is generated by the potential difference between the voltage applied to the corona wire 5a and the voltage applied to both the common electrode 4a and the control electrode 4b, and its certain portion enters the ion transmissive apertures 4c. If the potential difference between the aforementioned common and control electrodes 4a and 4b so as to increase the ion flow to enter those ion transmissive apertures 4c, the electric field becomes such an equi-potential plane as is indicated at solid lines in FIG. 2, with the trajectory of the ions indicated by broken lines. If that potential difference is reduced, the ion flow to enter the ion transmissive apertures 4c is reduced and the ion flow to transmit therethrough is accordingly reduced. In order to increase the reproducing speed, therefore, it is necessary to raise the potential difference between the common electrode 4a and the control electrode 4b. In this case, a portion of the ion flow having entered the ion transmissive apertures 4c is caught by the insulating layer between the common and control electrodes 4a and 4b, and a considerable portion is attracted by the control electrode 4b (which has a thickness of about 20 .mu.m) so that the transmission factor of the ion flow is reduced to a lower level than 10%. Here, the ion flow transmission factor means the ratio of the electric current due to the ion flow having transmitted through those ion transmissive apertures 4c to the electric current due to the ion flow having entered the ion transmissive apertures 4c of the ion flow modulating electrode.
In order to increase the reproducing speed of the electrostatic reproducing method using the ion flow modulating electrode, on the other hand, there has also been proposed a method which uses an ion generating source (which makes use of a high-frequency discharge and a spark discharge, for example) other than the ion generator using the corona wire. The apparatus according to the method proposed has more complicated structure than that using the corona wire. As another method, moreover, there has been proposed a method (which is disclosed in Japanese Patent Application No. 154,632/1980), in which a condenser electrode for converging the ion flow to enter the opening of the ion flow modulating electrode is provided, a method (which is disclosed in Japanese Patent Application No. 158,067/1980), in which the electrode layer of the ion flow modulating electrode at the side of the reproducing member is made thin so as to increase the ion transmission rate, or a method (which is disclosed in Japanese Patent Application No. 168,068/1980), in which an ion trajectory correcting electrode is interposed between the ion flow modulating electrode and the reproducing member.